1. Technical Field of the Invention
This invention relates generally signal processing, and more particularly to class-D amplifiers.
2. Description of Related Art
Integrated circuits are used in a wide variety of electronic equipment, including portable, or handheld, devices. Such handheld devices may include personal digital assistants (“PDA”), compact disc players, MPEG-1 Layer 3 digital audio (“MP3”) players, digital video disc players, AM/FM radio, a pager, cellular telephones, computer memory extension (commonly referred to as a thumb drive), etc. For example, an MP3 player may include one or more integrated circuits to support the storage and playback of digitally formatted audio (e.g., formatted in accordance with an MP3 specification), to interface with a computer, to generate a power supply voltage, and to render digitally formatted audio data audible.
The rendering of digitally formatted audio data into audible signals involves decoding the formatted audio data into digital audio signals, converting the digital audio signals into analog audio signals, and amplifying the analog audio signals. To provide a desired level of amplification, power efficiency, and compact size, class-D amplifiers are typically used to amplify the analog audio signals. While class-D amplifiers exhibit these positive characteristics, total harmonic distortion (“THD”) of a class-D amplifier varies with load, temperature, and/or other operating parameters.
As such, to provide high quality amplification of audio signals, the THD of class-D amplifiers needs to be compensated. There are a variety of techniques for THD compensation, but such techniques are generally static and do not effectively compensate noise introduced on the power supply. For instance, a static solution generates a calibration value at the start up the amplifier, where the calibration value is based on the start up impedances of the switching transistors and the start up inductor current. Thus, as the impedances of the switching transistors and the inductor current vary over time and temperature, the statically determined calibration value is less than optimal for compensating THD. In addition, the static solution does not account for transient noise and/or power supply noise.
Therefore, a need exists for a dynamic method and apparatus to minimize and/or compensate THD, transient noise, and/or power supply noise, of class-D amplifiers.